1. Field of the Invention
The present invention relates to a multiple coated composite steel strip having an excellent resistance to corrosion and a superior weldability and processability.
More particularly, the present invention relates to a multiple coated composite steel strip having a specific composite coating layer comprising metal plating layer formed on a steel strip substrate, a chromate coating layer formed on the metal plating layer, and an organic resin layer formed on the chromate coating layer and containing very fine colloidal silica particles.
2. Description of the Related Art
Various metal-plated steel strips, for example, electrically zinc-plated steel strips, galvanized steel strips, and various alloy-plated steel strip, are widely used in the manufacture of cars, electric appliances, and building materials.
Under these circumstances, the need for a composite steel strip having a surface coating layer providing an enhanced corrosion resistance is great, and will become greater in the future.
For example, in the home electric appliance industry, steel strips having a high corrosion resistance which can be used without a surface treatment or application of a coating are needed, to enable the manufacturer to omit the above-mentioned surface treatment or coating procedures and thus decrease the cost of producing the electric appliances.
Also, in the automobile manufacturing industry, due to significant changes in environment, cars are now exposed to a more severe corrosive environment, and thus, from the viewpoint of safety, there is a great demand for various surface treated or coated steel strips having a excellent resistance to corrosion due to, for example, rock salt powder sprinkled on roads to prevent icing of the road surface, as practiced in North America and North Europe, or acid rain caused by SO.sub.2 gas discharged from chemical plants into the atmosphere.
Various attempts have been made to meet the above-mentioned demands, wherein, in general, an increase of the corrosion resistance of the steel strip was effected by plating the steel strip with zinc.
The electrical zinc-plated or galvanized steel strip exhibits an enhanced corrosion resistance due to a self-sacrificing corrosion effect of the zinc-plating layer, and therefore, the intensity of the corrosion resistance of the zinc-plated steel strip is increased by increasing the amount or thickness of the zinc plating layer. Nevertheless, this increase in the amount of the zinc plating layer produces an undesirable increase in the cost for producing the zinc-plated steel strip, and results in a reduced processability, weldaility, and productivity of the zinc-plated steel strip. Also, it should be noted that, in general, the zinc-plated steel strip exhibits a poor bonding property to paints or lacquers.
To enhance the corrosion resistance of the zinc-plated steel strip, various types of alloy-plated steel strips were produced. The alloys used for plating these steel strips include, Zn-Ni alloys, Zn-Ni-Co alloys, Zn-Ni-Cr alloys, Zn-Fe alloys, Zn-Co alloys, and Zn-Mn alloys. These alloy-plated steel strips exhibited an enhanced corrosion resistance of 3 to 5 times that of the conventional zinc-plated steel strip, but when left in the open air for a long time or repeatedly sprayed with water or a salt solution, formations of red rust or white rust occurred on these conventional alloy-plated steel strips.
In another attempt to enhance the corrosion resistance, the metal-plated steel strip was further subjected to a chromate treatment. This attempt was quite effective but, when exposed to a high humidity, high temperature air atmosphere or to a salt-containing air atmosphere for about 100 to 150 hours, white rust was generated on the chromate-treated steel strip.
In still another attempt to enhance the corrosion resistance, the zinc-plated and chromate-treated steel strip was further coated with various types of resinous coating materials. This type of coated steel strip is known as an easy pre-coated steel strip and has limited use in practice.
For example, Japanese Unexamined Patent Publication Nos. 58-210.190 and 58-210,192 disclose a weldable composite steel strip having a metal plating layer consisting of a single zinc alloy-plating layer or two zinc alloy-plating layers and formed on a steel strip substrate, a chromate coating layer formed on the alloy-plating layer and a resinous paint-coating layer formed on the chromate coating layer and containing a resinous matrix material and electroconductive material, for example, Zn, Al, Sn, Fe, Ni, Co, Cr, and Mn, and exhibiting an enhanced weldability, paint-bonding property, and corrosion resistance.
The resinous paint-coating layer has a thickness of 5 to 15 .mu.m. The resinous matrix material comprises at least one member selected from synthetic resins, for example, polyacrylic resins, epoxy resins, polyurethane resins, polyvinylchloride resins, polyester resins, polybutadiene resins, alkyd resins, polystyrene resins and phthalic acid resins, and natural resins.
Japanese Unexamined Patent Publication N0. 58-224,174 discloses a process for producing a composite steel strip having enhanced resistances to corrosion and rust, comprising plating a steel strip substrate with a zinc alloy, subjecting the plated steel strip to a coating type chromate treatment, drying the chromate-treated steel strip without washing with water, and then treating the chromate-treated steel strip with a solution of a composite organic silicate resin.
This Japanese publication 174 discloses that, when the amount of the resultant composite organic silicate resin coating layer is controlled to a level of 4.0 g/m.sup.2 or less, the resultant coated composite steel strip exhibits a satisfactory spot weldability. Also, this publication teaches that the composite organic silicate resin can be prepared by reacting a water-soluble or dispersable polymeric material with a water-dispersable silica in the presence of a silane compound at a temperature of 50.degree. C to 90.degree. C.
Japanese Unexamined Patent Publication No. 59-116,397 discloses a composite steel strip having two electroplated Ni-Zn or Fe-Zn alloy first layers formed on two surfaces of a steel strip substrate, a thin electroplated alloy second layer, for example, an Fe-Zn alloy layer, formed on one of the first layers, and a resinous layer formed in a thickness of 3 to 10 .mu.m on the other one of the first layers, and comprising a mixture of an epoxy resin or polyester resin and an electroconductive pigment, or an electroconductive pigment-free epoxy resin or polyester resin. The composite steel strip exhibited enhanced resistances to corrosion and to rust. The one surface of the composite steel strip exhibited a high corrosion resistance and a high paint-bonding property and the other surface thereof exhibited an enhanced spot-weldability and workability.
Japanese Examined Patent Publication No. 61-36,587 discloses a process for treating surfaces of a steel strip comprising forming a chromate coating layer on an electroplated zinc layer on a steel strip substrate, coating the electroplated surface with a specific aqueous solution containing a carboxylated polyethylene resin and a colloidal silica, and drying the solution layer. The resultant dried composite steel strip exhibited an enhanced resistance to finger marking, and a higher paint-bonding property, hardness and corrosion resistance.
Japanese Unexamined Patent Publication No. 60-149,786 discloses a method for treating a surface of a metal plated steel strip, similar to that disclosed in Japanese Examined Patent Publication No. 61-36,587, except that the electroplated zinc layer is replaced by an electroplated zinc alloy layer, and the resinous material is replaced by a water-soluble resinous material. This method is effective for further increasing the corrosion resistance and solvent resistance of the composite steel strip.
Japanese Unexamined Patent Publication No. 61-167,545 discloses a paint-coated weldable composite steel strip having a zinc alloy-plating layer formed on a steel strip substrate, a chromate layer formed on the zinc alloy-plating layer, and a paint layer containing zinc powder and at least one member selected from hard metal powder and hard carbide powder. The composite steel strip exhibited an enhanced corrosion resistance and weldability.
The above-mentioned known composite steel strip are known as organic coated composite steel strips and have enhanced corrosion and rust resistances and other improved properties.
However, the above-mentioned types of organic coated composite steel strips do not always have satisfactory corrosion and rust resistances, especially when exposed to severe corrosive circumstances for a long time, or weldability and/or workability (especially continuous press workability and bead-workability).
For example, in the coated composite steel strip disclosed in Japanese Unexamined Patent Publication Nos. 58-210,190 , 58-210,192 and 59-116,397, the electroconductive pigment contained in a resinous coating layer causes an undesirably reduced evenness of the electrodeposited paint coating layer surface and an undesirably increased powdering and buildup of the coated composite steel strip when pressed, and thus an undesirably decreased press-workability. Also, in the coated composite steel strip disclosed in Japanese Unexamined Patent Publication No. 59-116,397, even when the resinous coating layer does not contain the electroconductive pigment, the resultant composite steel strip exhibits an unsatisfactory weldability and press-workability (that is, an increased buildup), the surface of the resinous coating layer exhibits an unsatisfactory lubricity and a poor paint coating property, and the paint coating layer formed on the resinous coating layer has an uneven surface.
Further, in the coated composite steel strip disclosed in Japanese Unexamined Patent Publication of 58-224,174, the composite organic silicate resin coating layer sometimes has an uneven thickness. Even if the unevenness of the thickness is very small, the uneven composite organic silicate resin coating layer results in an uneven surface of a paint-coating layer formed thereon.
Furthermore, the coated composite steel strips disclosed in Japanese Examined Patent Publication No. 61-36,587 and in Japanese Unexamined Patent Publication No. 61-167,545 exhibit an unsatisfactory weldability and surface lubricity. Accordingly, when pressed, the composite steel strips sometimes exhibit an increased galling, powdering and buildup, and thus a degraded press-workability. Also, when the paint is electrodeposited, the resultant paint layer has an uneven surface and exhibits a poor lubricity.
As stated above, the coated composite steel strips do not have a satisfactory corrosion and rust resistance and weldability and/or workability, and therefore, are not always usable as rust-resistant steel strips for car bodies.